1.such a problem exists because adequate adequate measures for preventing it ____taken inthe past.为什麽用were not 不用had been not?
2.li Gang often told stories to us and he ______with enthusiasm.用was always listened to为什麽不用whom?

我来解释一下，不一定正确，仅供参考：
1.such a problem exists because adequate adequate measures for preventing it ____taken inthe past.为什麽用were not 不用had been not?
之所以选用“were not ”而不选用“had been not”，其原因主要是介词短语“in the past”作状语之故。表明被修饰的动作发生在过去（的某个时刻）。
2.li Gang often told stories to us and he ______with enthusiasm.用was always listened to为什麽不用whom?
纵观该句子，属于用and连接的并列句。这里的“he”是主语，代指“li Gang ”，其后应跟随系词或动词，不应用引导词“whom”，所以选择“was always listened to”而不选择“whom”。